Method lfor preparing soluble dietary fiber from corn hull

ABSTRACT

The present invention relates to a high yield process for producing soluble dietary fiber from corn hull. More specifically, the present invention comprises removing starch and protein with enzymes from corn hull which is by-product of wet-milling process for production of corn starch, extracting the resultant with alkaline solution to form a alkaline extract, treating the alkaline extract with enzymes, and drying the enzyme-treated solution, to produce dietary fiber with low viscosity and containing hemicellulose as a major component at high yield.

BACKGROUND OF THE INVENTION

[0001] (a) Field of the Invention

[0002] The present invention relates to a method for preparing solubledietary fiber from corn hulls.

[0003] (b) Description of the Related Arts

[0004] In general, dietary fiber can be defined as “all the componentsof a plant that are resistant to digestion by human digestive enzymes.”

[0005] Dietary fiber can be classified into a water-insoluble group anda water-soluble group. It has been known that soluble dietary fiber hasa good effect on suppression of increase of serum cholesterol level. Themechanism of soluble dietary fiber to improve lipid metabolism issupported by the functions that soluble dietary fiber inhibits lipidabsorption in the digestive tract and accelerates excretion of bileacid, and that the short chain fatty acids, in particular propionic acidproduced from fermentation of the fiber in the colon, suppresses thesynthesis of cholesterol. Well-known insoluble dietary fibers includecellulose, lignin, and the like, and soluble dietary fibers include Guargum, Arabic gum, pectin, hemicellulose, and the like. In addition,indigestible components that are contained in cell walls and cellcontents of plants such as grains have been called crude fiber, and theyinclude hulls of grains such as rice, corn, and beans.

[0006] Corn hulls, produced from wet milling in the preparation processof starch, can be used as a good source of dietary fiber, because thecorn hulls contain a lot of dietary fiber, and in particular morehemicellulose than hulls of other grains. Hemicellulose is apolysaccharide that consists of complicatedly connected monomers, andthat is a component of cell walls of plants except for cellulose andpectin. Hemicelluloses are divided into two groups: water-insolublehemicellulose A, and water-soluble hemicellulose B. Solublehemicellulose B includes a main component of arabinoxylan which consistsof xylose and arabinose. Arabinoxylan has a beta-1,4-linkage that cannotbe hydrolyzed by human digestive enzymes, and thus can be a good dietaryfiber.

[0007] There are many suggestions on preparation methods for dietaryfiber from grain hulls.

[0008] Japanese Laid-Open Patent Publication Pyung 1-242540 described aprocess for preparing dietary fiber powder. Corn hulls were treated withglucoamylase for 24 to 40 hours to remove starch, extracted at roomtemperature for 18 hours after adding 0.5N NaOH solution (2%), andcentrifuged. The extract was then treated with trichloroacetic acid andcentrifuged again to precipitate protein.

[0009] The resultant solution was contained in a cellophane tube fordialysis in water for 3 days, and was then left after adding ethanol toobtain precipitates, which were subsequently recovered (finalyield=13%).

[0010] Japanese Laid-Open Patent Publication Pyung 3-209331 disclosed apreparation method of dietary fiber powder. Corn hulls obtained from wetmilling in a preparation process of corn starch were mixed in ahomogenizer with the addition of water, and then filtered with a sieve.The resultant was extracted with a NaOH or Ca(OH)₂ solution,centrifuged, and neutralized to pH 7. The neutralized solution was driedin a drum dryer, or it was treated with alkaline xylase, purified inseveral processes, and then dried in a spray dryer to produce powder.

[0011] Japanese Patent Publication Pyung 6-11764 disclosed a method ofto preparing soluble hemicellulose. Defatted rice bran was treated withheat-resistant amylase in hot water to remove starch, and was thenextracted under an alkaline condition of pH 10 or more, or an acidiccondition of pH 3 or more, to produce a solution containing a solublehemicellulose B fraction. Insoluble components were removed byneutralizing the resultant solution and performing ultra-filtration. Theresultant was purified to produce soluble hemicellulose (finalyield=3.5˜8%).

[0012] WO 98-40413 disclosed a preparing method for corn fiber gum withan improved color and low viscosity. Specifically, the corn hulls weredestarched with heat-resistant amylase, extracted with a hydrogenperoxide, NaOH, and a Ca(OH)₂ solution, and then ultra-filtrated toremove insoluble components such as a hemicellulose A fraction. Theresultant solution was concentrated and dried in a spray dryer.Otherwise the solution was precipitated with alcohol afterconcentration.

[0013] The preparing methods for soluble dietary fiber from grain hullsas mentioned above have many disadvantages, as follows. The fine fiberis removed in the removing process of starch and protein, therebylowering the yield of fiber. Because Ca(OH)₂ with low water-solubilityis used in the alkaline extracting process, the content of ash is highin the final sample. Thus, many purification processes are required, andthe extraction yield is decreased. In addition, due to centrifugationafter alkaline extraction, the extract solution cannot be recoveredmaximally. Thus, the final yield of soluble fiber decreases, and themaximal recovery rate according to prior arts is only 13%. Use oforganic solvent such as alcohol makes the preparation processcomplicated and the industrialization of the process difficult.

SUMMARY OF THE INVENTION

[0014] The object of the present invention is to provide a preparingmethod for soluble dietary fiber that contains hemicellulose as a maincomponent and has high transparency and low viscosity_(T) at a highyield, from corn hulls.

DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention provides a high yield method of pre paringsoluble dietary fiber with a low viscosity.

[0016] The method of the present invention comprises the steps of:

[0017] (i) removing starch and protein from corn hulls;

[0018] (ii) extracting starch- and protein-removed the corn hulls withan alkaline solution, and filtering the alkaline extract through afilter cloth;

[0019] (iii) treating the filtrate of step (ii) with cellulase andcellobiase;

[0020] (iv) treating the solution reacted with enzyme of step (iii) withan adsorbent, and then filtering it through a membrane filter; and

[0021] (v) purifying the filtrate.

[0022] As desired, to further improve the transparency andfilterability, step (iii) further comprises a step of treatment withxylanase. Preferably, before the to enzymatic reaction, the alkalineextract is further desalted and decolorized with an ion exchange resin.

[0023] The corn hulls used in the present invention are commerciallyavailable. As long as the corn hulls are commercially available, allcorn hulls can be used without considering their quality level.

[0024] In the present invention, the generally known methods forremoving starch and protein can be used. For example, in the enzymaticmethod, the corn hulls can be treated with a starch degrading enzymesuch as amylase and glucoamylase, and protease. Treatment with thestarch degrading enzyme and the protease can be done simultaneously orsequentially.

[0025] In the invention, after removal of starch and protein from cornhull s, the resultant can be filtered through a filter cloth, therebyincreasing the recovery rate of corn hulls.

[0026] To extract hemicellulose, which constitutes 70% or more of cornhulls, the corn hulls are stirred with the addition of an alkalinesolution at a high temperature. The alkaline solution can be NaOH, or amixture of NaOH and Ca(OH)₂. When only the sodium hydroxide is used, ahigher yield, a higher content of dietary fiber, and more advantages inthe preparation process can be expected compared to using sodiumhydroxide and calcium hydroxide together. In considering the efficientextraction of hemicellulose, it is preferable to use sodium hydroxide ata low concentration. The maximal recovery rate of the extract can bereached by filtering the extract with a filter cloth after alkalineextraction.

[0027] After neutralizing the extract with acid, the resultant solutionis simultaneously treated with cellulase and cellobiase, or cellulase,cellobiase and xylanase. The addition of cellulase, cellobiase, andxylanase makes the extract less viscous, more transparent, and morefilterable, thereby making the production process more advantageous.

[0028] In addition, when the alkaline extract is treated by desaltingand decolorization with a cation or anion exchange resin before theenzyme treatment, it is possible to reduce the amount of the enzymewhich is required in the following step to produce the same quality ofsoluble dietary fiber.

[0029] The solution obtained from the enzyme reaction is treated with anadsorbent such as activated carbon, filtered by membrane filtration,treated with an ion exchange resin, concentrated, and dried, to producethe water-soluble dietary fiber.

[0030] The preparing method of the soluble dietary fiber from corn hullsis more specifically described in the following.

First Step: Removal of Starch and Protein from Corn Hulls

[0031] Corn hulls obtained from corn starch production are dried toabout 5% of water content. The dried corn hulls are mixed with distilledwater in the amount of 10 to 20 times, preferably 15 times by weight ofthe corn hulls, and the pH is adjusted to pH 5.8 to 6.0 by the additionof a 3 to 5% NaOH standard solution. After the resultant solution isheated by stirring in a water bath so that the temperature of thesolution is 90 to 100° C., the solution is stirred for 1 to 5 hours withthe addition of alpha-amylase in the amount of 0.05 to 5%, preferably0.1 to 3% to the dried corn hulls, and then filtered through a filtercloth, and the corn hulls are then sufficiently washed with water. Thealpha-amylase, for example Termamyl (Novo Nordisk LTD.), is preferablyheat-resistant. The filter cloth can be a generally used one that ismade from polyester and polyamide with an internal pore size of 36 to100, and preferably 44 to 53 micrometers.

[0032] Destarched corn hulls are then suspended in distilled water inthe same ratio as above, and the pH of the solution is adjusted to pH7.0 by the addition of a sodium hydroxide solution. After the resultantsolution is heated by stirring in a water bath so that the temperatureof the solution is 45 to 55° C., it is stirred for 1 to 5 hours with theaddition of protease in the amount of 0.05 to 5%, preferably 0.2 to 2%,filtered through the filter cloth, and washed by the same method asabove, to obtain starch- and protein-removed corn hulls. The proteasesinclude fungi enzymes such as flavourzyme derived from Asperfillusoryzae, alkalase derived from Bacillus licheniformis, and the like. Whenfiltration is performed with the filter cloth, the recovery rate of cornhulls is higher than with centrifugation, which is shown in Example 1.

Second Step: Alkaline Extraction

[0033] The hemicellulose is extracted from the starch- andprotein-removed corn hulls with an alkaline solution. The corn hullsfiltered can be used directly, or after being dried to some extent. Theresultant corn hulls can be mixed with a sodium hydroxide solution at alow concentration of 0.1 to 3%, preferably 0.5 to 0.7% in the amount of15 to 25 times, preferably 20 to 25 times by weight of the corn hulls.Then the hemicellulose is extracted by heating the mixture in a waterbath, and stirring at 70 to 90° C. for 1 to 5 hours. The resultant isthen cooled to room temperature, and filtered by vacuum filtration witha filter cloth to is produce the extract. The same type of filter clothas used in the first step can be used. As shown in Example 3, whenfiltering with a filter cloth, the recovery rate and transparency of thealkaline extract increases, thereby improving the filterability andfinal yield in the following process, compared to centrifugation.

Third Step: Desalting and Decolorization

[0034] After preparing the alkaline extract, the solution can bedirectly treated with enzymes such as cellulose, cellobiase, andxylanase, etc. However, before the enzyme treatment, the alkalineextract can be further desalted and decolorized with an ion exchangeresin. This case has advantages in that a smaller amount of enzymes isrequired in the enzyme reaction, and it results in a higher yield thanwith direct enzyme treatment without desalting and decolorization.

[0035] The ion exchange resin which is generally used for preparation ofstarch sweetner can be used in the desalting and decolorizing step. Asexamples, a strongly acidic cation exchange resin or a weakly basicanion exchange resin can be used. The ion exchange resins can be addedin the amount of 1 to 10 times, and preferably 4 times the volume of thedried corn hulls. For example, a cation exchange resin including thestrongly acidic styrene resin SK1B can be used, and the anion exchangeresin s that can be used include a strongly basic Cl-type and a weaklybasic OH-type styrene resin.

Fourth Step: Enzyme Treatment

[0036] The pH of the alkaline extract, or the desalted and decolorizedalkaline extract, is adjusted to pH 4.0 to 5.5, preferably 4.7 to 5.0,which is optimum for enzymes. Then the temperature of the resultantsolution is adjusted to an appropriate temperature for the activeenzymes by heating it in a water bath, and it is treated with cellulaseand cellobiase while stirring. Preferably, xylanase may be used fortreatment together with the cellulase and cellobiase, thereby obtainingan enzyme hydrolysate with an improved filterability, low viscosity, andhigh transparency.

[0037] In regard to the preferred dosage content of the enzymes, whenthe alkaline extract is directly treated with enzymes, the dosage ofcellulase and cellobiase are the same, at 0.1 to 5%, preferably 0.1 to3%, respectively. When the amount of the enzymes is lower, it isdifficult to perform the following processes, such as the filteringstep. When the amount of the enzymes is higher, the reaction time isreduced, but the production costs increase.

[0038] When the alkaline extract is treated with enzymes after adesalting and decolorizing step, 10 to 90 parts by weight of cellobiaseto 100 parts by weight of cellulase can produce a final product with aquality level equivalent to that of an enzyme reaction on an alkalineextract lacking the desalting and decolorizing step. The content ofcellulase can be 0.1 to 5 wt % to weight of lo dried corn hulls. Thecellobiase can be used in the amount of 0.1 to 5 wt %, preferably 0.1 to3 wt %, to weight of the dried corn hulls.

Fifth Step Adsorbent Treatment and Filtration

[0039] The enzyme reaction mixture is treated with an adsorbent, andthen filtered. The adsorbents include activated carbon, an adsorbingresin such as polystyrene, and the like. To obtain a transparentextract, the filtration process can be performed with a membrane filterwith a pore size of 0.5 micrometer or less, and preferably 0.45 to 0.2micrometer or less.

Sixth Step: Final Purification, Concentration and Drying

[0040] The final purification of the extract can be performed bydesalting and decolorizing with an ion exchange resin which is generallyused in the preparation process of dietary fiber. For example, the ionexchange resins include a strongly acidic cation exchange resin, aweakly basic anion exchange resin, and a mixed ion exchange resin. Themixed resin can be a mixture of an activated strongly acidic cationexchange resin and a strongly basic anion exchange resin, in the volumeratio of 1:2. For example, the cation exchange resins include a stronglyacidic styrene resin SK1B, and the anion exchange resins include astrongly basic Cl-type styrene resin and a weakly basic OH-type styreneresin.

[0041] The finally purified enzyme hydrolysate is concentrated to a 10%solution under vacuum, and is then freeze-dried or spray-dried toproduce powder. The employment of the third step in the preparationprocess for the dietary fiber can omit the purification process with thecation exchange resin or to anion exchange resin, resulting insimplifying the preparation process.

[0042] The following examples are intended only to illustrate theinvention and are not intended to limit the scope of the invention asdefined by the claims.

EXAMPLE 1

[0043] Dried corn hulls (2.8% of water content) were added to distilledwater in the amount of 15 times by weight of the dried corn hulls, andthe pH was adjusted to 5.8 with 1N NaOH while mixing with a mechanicalstirrer. The mixture was heated in a 95° C. water bath, reacted for 2hours with the addition of heat-resistant alpha-amylase (Novo NordiskLtd., Termamyl 120 LS, Denmark) in the amount of 2.0% to the dried cornhulls, filtered by a polyester filter cloth (Samsung Canvas, 55-5528,Korea), and washed with distilled water.

[0044] The destarched corn hulls were added to distilled water in theamount of 15 times by weight of the dried corn hulls, and the pH wasadjusted to 7.0 with 1N NaOH while mixing with a mechanical stirrer. Themixture was reacted with the addition of protease (Novo Nordisk Ltd.,Flavourzyme, Denmark) in the amount of 2.0% to the dried corn hulls for2 hours in a 50° C. water bath, filtered by a polyester filter cloth(Samsung Canvas, 55-5528, Korea), and washed with distilled water. Theresultant was dried in an oven at 50° C., to produce starch- andprotein-removed corn hulls. The yield of corn hulls is shown in Table 1.

EXAMPLE 2

[0045] The same method as in Example 1 was used, except that instead ofFlavourzyme, alkalase (Novo Nordisk Ltd., Alkalase, Denmark) in theamount of 2.0% to the dried corn hulls was reacted for 2 h ours in a 55°C. water bath, to produce starch- and protein-removed corn hulls. Theyield of corn hulls is shown in Table 1.

Comparative Example 1

[0046] The same method as in Example 1 was used, except that after thetreatment of alpha-amylase and protease, the resultant solution wascentrifuged at 3,000 rpm instead of filtering with the filter cloth, toproduce starch- and protein-removed corn hulls. The yield of corn hullsis shown in Table 1. TABLE 1 The yield of starch- and protein-removedcorn hulls Yield of corn hulls (%)¹ Remark Vacuum filtration 73.7Polyester filter cloth (Example 1) Vacuum filtration 74.2 Polyesterfilter cloth (Example 2) Centrifugation 65.0 Vacuum filtration

EXAMPLE 3 Preparation of Alkaline Extract

[0047] 37 g of starch- and protein-remvoed corn hulls prepared accordingto Example 1 were mixed with 1 L of a 0.5% NaOH solution, and stirred inan 80° C. water bath for 3 hours, and filtered through a filter cloth toproduce a primary alkaline extract. The yield and transparency asmeasured with a spectrophotometer are shown in Table 2

Comparative Example 2

[0048] The corn hulls obtained in Comparative Example 1 were treatedwith an alkaline solution as in Example 3, and then they werecentrifuged instead of filtered, to produce an alkaline extract. Theyield and transparency as measured with a spectrophotometer are shown inTable 2. The results show that the yield and transparency with vacuumfiltration are higher than with the centrifugation method. TABLE 2 Yield(%)¹ Transparency (%)² Treatment type Example 3 55.3 62.7 Vacuumfiltration with polyester filter cloth Comparative 47.6 34.3Centrifugation Example 2

EXAMPLE 4 Enzymatic Treatment

[0049] The pH of 1.2 L of the primary extract obtained in Example 3(27.7 g of solid content) was adjusted to pH 4.8 by the addition of 10%HCl, and cellulase (Novo Nordisk Ltd, Celluclast, Denmark) andcellobiase (Novo Nordisk Ltd., to Novozyme 188, Denmark) were added inthe amount of 3% by weight of the dried corn hulls in a 50° C. waterbath. Then, the viscosity, filterability, and transparency of theresultant solution were measured, and the results are shown in Table 3.To compare the results, the above process was repeated, except that only3.0% by weight of the cellulase was used.

[0050] In addition, the pH of the same primary extract as above wasadjusted to pH 4.8 by the addition of 10% HCl, and cellulase (NovoNordisk Ltd, Celluclast, Denmark), cellobiase (Novo Nordisk Ltd.,Novozyme 188, Denmark), and xylanase (Biocatalyst Co., Depot 333p, UK)in the amount of 3% by weight of the dried corn hulls were addedsimultaneously to the extract, reacted in a 60° C. water bath for 3hours. Then, the viscosity, the filterability, and the transparency ofthe resultant solution were measured, and the results are shown in Table3.

[0051] 2.5 g of activated carbon (Norit Co., KB-B, Holland) were addedto an enzyme hydrolysate of three enzymes, it was heated to 95° C. for30 seconds, and cooled to room temperature. The resultant was primarilyfiltered into filter paper (Advantec Co., Toyo 5A, Japan), and thensecondly into filter paper (Whatman International Ltd., GF/B, UK) in aglass filter under vacuum. Then, the resultant was filtered through amembrane with a pore size of 0.45 micrometers (Gelman Co., Metricel,USA) and a membrane with a pore size of 0.2 micrometers (Gelman Co.,Super-200, USA).

[0052] The obtained filtrate was treated with a strongly acidic cationexchange resin (Samyang Co., SK-1B, Korea), a weakly basic and stronglybasic anion exchange resin (Samyang Co., WA 30; PA 408, Korea), and aresin mixture (strongly acidic cation exchange resin and strongly basicanion exchange resin in the ratio of 1:2) at each step, it was stirredin a 40° C. water bath for 1.5 hours, filtered, and desalted anddecolorized.

[0053] Finally, the resultant was concentrated with a vacuum evaporator(EYELA, NE-1V, Japan), so that the concentration of the soluble dietaryfiber was measured to be 10% (w/w). The final yield of soluble dietaryfiber to raw corn hulls was 23.1%. As a result of measurement of thefiber with a Prosky-AOAC method, the content of soluble dietary fiberwas 91.9%, and the results of typical analysis are shown in Table 4.TABLE 3 Viscosity, Filterability, and Transparency of enzymehydrolysate¹ depending on the condition of the enzyme treatmentCondition of Viscosity² Filtering rated³ Transparency⁴ enzyme treatment(cps) (second) (%) Cellulase 15.0 Not filtered  6.4 Cellulase +cellobiase 10.0 201 64.7 Cellulase + cellobiase +  8.0  55 95.5 xylanase

[0054] TABLE 4 Analysis of components, pH, viscosity and transparency ofsoluble dietary fiber prepared from corn hulls Item Result Analyticalmethod Water (%) 5 or less Oven drying method Crude protein (%) 0.7Kjeldahl method Crude lipid (%) 0.1 Soxhlet method Crude ash (%) 0.4 orless Incineration method pH 3.0 ˜ 5.0 pH meter (at 5% concentration)Viscosity  8 ˜ 12 Viscometer (LVF type) (cps, at 5% concentration)Transparency 95 or more Spectrophotometer (T %, at 5% concentration)

EXAMPLE 5 Desalting and Decolorizing Process

[0055] To perform the desalting and decolorizing process, the pH of 1.2L of primary extract obtained in Example 3 (27.7 g of solid content) wasadjusted to pH 6.0 by adding 10% HCl, and 200 mL of the strongly acidiccation exchange resin (Samyang Co., SK-1B, Korea) and the weakly basicanion exchange resin (Samyang Co., WA 30, Korea) were addedsequentially, it was stirred in a 40° C. water bath for 1.5 hours, andthen filtered.

[0056] The pH of the resultant solution was adjusted to pH 4.8,cellulase (Novo Nordisk Ltd, Celluclast, Denmark) and cellobiase (NovoNordisk Ltd., Novozyme 188, Denmark) were added in the amount of 2% and0.4% by weight of the dried corn hulls respectively, and it was reactedfor 3 hours in a 50° C. water bath. Then, the viscosity, thefilterability, and the transparency of the resultant solution weremeasured, and the results are shown in Table 5. To compare the results,the above process was repeated, except that only 2.0% of the cellulasewas used.

[0057] In addition, the pH of the same primary extract as above wasadjusted to pH 6.0, and then it was desalted and decolorized accordingto the method as mentioned above. Then, the pH of the resultant solutionwas adjusted to 4.8, cellulase (Novo Nordisk Ltd, Celluclast, Denmark),cellobiase (Novo Nordisk Ltd., Novozyme 188, Denmark), and xylanase(Biocatalyst Co., Depol 333p, UK) were added simultaneously in theamount of 2.0%, 0.4%, and 2.0% by weight of the dried corn hullsrespectively, and it was reacted in a 55° C. water bath for 3 hours.Then, the viscosity, the filterability, and the transparency of theresultant solution were measured, and the results are shown in Table 5.

[0058] 2.5 g of the activated carbon (Norit Co., KB-B, Holland) wereadded to an hydrolysate of three enzymes, it was heated to 95° C. for 30seconds, and cooled to room temperature. The resultant was primarilyfiltered into filter paper (Advantec Co., Toyo 5A, Japan), and thensecondly into filter paper (Whatman International Ltd., GF/B, UK) in aglass filter under vacuum. Then, the resultant was filtered into amembrane with a pore size of 0.45 micrometers (Gelman Co., Metricel,USA) and a membrane with a pore size of 0.2 micrometers (Gelman Co.,Super-200, USA).

[0059] The obtained filtrate was added to 200 mL of a resin mixture of astrongly acidic cation exchange resin (Samyang Co., SK-1B, Korea) and aweakly basic anion exchange resin (Samyang Co., WA 30; PA 408, Korea) inthe volume ratio of 1:2, and stirred in a 40° C. water bath for 1.5hours to complete the final purification process.

[0060] Finally, the resultant solution was concentrated with a vacuumevaporator (EYELA, NE-1V, Japan), so that the concentration of thesoluble dietary fiber was measured be to 10% (w/w). The final yield ofsoluble dietary fiber to raw corn hulls was 24.5%. As a result ofmeasurement of the fiber with a Prosky-AOAC method, the content ofsoluble dietary fiber was 92.1%, and the results of typical analysis areshown in Table 6. TABLE 5 Viscosity, Filterability, and Transparency ofenzyme hydrolysate¹ depending on the condition of the enzyme treatmentCondition of Viscosity² Filtering rated³ Transparency⁴ enzyme treatment(cps) (second) (%) Cellulase 15.0  Not filtered  6.4 Cellulase +cellobiase 9.5 190 68.0 Cellulase + cellobiase + 9.0  57 96.1 xylanase

[0061] TABLE 6 Analysis of components, pH, viscosity, and transparencyof soluble dietary fiber prepared from corn hulls Item Result Analyticalmethod Water (%) 5 or less Oven drying method Crude protein (%) 0.6Kjeldahl method Crude lipid (%) 0.1 Soxhlet method Crude ash (%) 0.5 orless Incineration method pH (at 5% concentration) 3.0 ˜ 5.0 PH meterViscosity  8 ˜ 12 Viscometer (LVF type) (cps, at 5% concentration)Transparency 95 or more Spectrophotometer (T %, at 5% concentration)

EXAMPLE 6

[0062] This example was produced according to the same method of Example4, except that xylanase derived from Aspergillus niger (Novo NordiskLtd., Shearzyme 500 L, Denmark) was used instead of xylanse. The finalyield of dietary fiber to raw corn hulls was 20.4%, and the content ofdietary fiber was 90.7%.

EXAMPLE 7

[0063] 200 g of corn hulls (5.7% of water content) were mixed withdistilled water so that the final concentration was 8%, 1N NaOH solutionwas added while mixing with an a mechanical stirrer, to a pH of 5.8. Themixture was heated in a 95° C. water bath, reacted with the addition ofthe heat-resistant alpha-amylase of Example 1 in the amount of 1.0%(v/w) to the dried corn hulls, for 2 hours, and filtered with a filtercloth. The destarched corn hulls were added to 3.0 L of distilled water,and the pH was adjusted to 7.0 with 1N NaOH solution. The mixture wasreacted with protease (Novo Nordisk Ltd., Flavourzyme, Denmark) in theamount of 1.0 (w/w) % to the dried corn hulls for 3 hours in a 50° C.water bath, filtered with a filter cloth, and dried in an oven at 50°C., to produce starch- and protein-removed corn hulls. The yield of cornhulls was 74.0%. To perform alkaline extraction, the starch- andprotein-removed corn hulls were mixed with 3 L of a 0.5% NaOH solution,and stirred in a 40° C. water bath for 24 hours and filtered through afilter cloth to produce a primary alkaline extract. The pH of theprimary extract was adjusted to pH 4.8 by addition of 10% of HCl, andcellulase (Novo Nordisk Ltd, Celluclast, Denmark) and cellobiase (NovoNordisk Ltd., Novozyme 188, Denmark) in the amount of 1.0% by weight ofthe dried corn hulls were respectively added, and reacted in a 50° C.water bath for 5 hours. After the reaction, 10% (w/w) of the activatedcarbon (Norit Co., KB-B, Holland) to corn hulls was added to thehydrolysate of three enzymes, it was heated to 95° C. for 30 seconds,and cooled to room temperature. The resultant was finally filtered witha membrane with a pore size of 0.45 micrometers (Gelman Co., Metricel,USA). As in the method of Example 3, the desalting and decolorizingsteps were performed with three steps of ion exchange resin treatment.The resultant solution was concentrated so that the final concentrationwas 10% (w/w), and then the final yield of soluble dietary fiber to theraw dried corn hulls was calculated. As a result, the yield was 21.5%,and the content of the fiber was 86.4%.

EXAMPLE 8

[0064] According to the method of Example 1, 300 g of corn hulls wereadded to distilled water in the amount of 15 times by weight of the cornhulls, and then starch and protein were removed with enzymesequentially. The yield of starch- and protein-removed corn hulls was68.6%.

[0065] Alkaline extraction was performed on the corn hulls with 5 L of0.5% NaOH, and then the resultant solution was divided into two groups.5% of Cellulase and 5% (v/w) of cellobiase to the dried corn hulls wereadded to one group, and reacted at 50° C. for 5 hours. 5% (v/w) ofCellulase, 5% (v/w) of cellobiase, and 5% (v/w) of xylanase to the driedcorn hulls were added simultaneously to the other group and reacted at60° C. for 5 hours. The two groups were treated according to thepurification process as above. Then, the concentrations of dietary fibersolution were adjusted to 5% (w/w) at room temperature, and theviscosities were measured. As a result, with the treatment of cellulaseand cellobiase, the viscosity was 10.0 cps. With the treatment ofcellulase, cellobiase, and xylanase, the viscosity was 9.0 cps. Forreference, Arabic gum (MSC Co., No. 10308, Korea), which is usually usedas an emulsion stabilizer for food, had a viscosity of 6.7 cps.

EXAMPLE 9

[0066] After the starch and protein were removed from the corn hullsaccording to the method of Example 1, the resultant was divided into twogroups. One group was treated with 0.5% of an alkaline mixture whichincluded NaOH and Ca(OH)₂ in the same ratio in the amount of 20 times byweight of corn hulls. The other group was treated with 0.5% of a NaOHsolution in the same amount, and reacted at 80° C. for 3 hours. Asdisclosed in Example 4, the resultant solutions were treated with thethree kinds of enzymes in the amount of 3% respectively, reacted,purified, and the yield was calculated. As a result, in the case oftreatment with NaOH and Ca(OH)₂, the yield was 15.1%, and the content ofdietary fiber was 72.7%. In the case of treatment with NaOH, the yieldwas 20.1%, and the content of dietary fiber was 84.3%.

EXAMPLE 10

[0067] After the starch and protein were removed from the corn hullsaccording to the method of Example 1, the resultant was divided intothree groups. The groups were treated with a 0.1%, 0.5%, and 1.0% NaOHsolution in the amount of 20 times by weight of corn hulls,respectively. As disclosed in Example 4, the resultant solutions weretreated with the three kinds of enzymes in the amount of 3%respectively, reacted, purified, and the yield was calculated. As aresult, when treating with 0.1%, 0.5%, and 1.0% NaOH, the yields were3.3%, 30.0%, and 33.4%, respectively. However, the treatment with 1.0%NaOH caused the formation of salt in a relatively large amount duringneutralizing process, resulting in a high load during the purificationprocess.

EXAMPLE 11

[0068] After the starch and protein were removed from the corn hullsaccording to the method of Example 1, the resultant was divided intothree groups. The groups were treated with a 0.5% NaOH solution in theamount of 20 times by weight of corn hulls for 1, 3, and 10 hours,respectively. As disclosed in Example 4, the resultant solutions weretreated with the three kinds of enzymes in the amount of 3%respectively, reacted, purified, and the yield was calculated. As aresult, when treatment time was 1, 3, and 10 hours, the yields were24.9%, 27.9%, and 26.7%, respectively.

EXAMPLE 12

[0069] After the starch and protein were removed from the corn hullsaccording to the method of Example 1, the resultant was divided intothree groups. The groups were treated with a 0.5% NaOH solution in theamount of 20 times by weight of corn hulls for 3 hours at 40, 60, and80° C., respectively. As disclosed in Example 4, the resultant solutionswere treated with the three kinds of enzymes in the amount of 3%respectively, reacted, purified, and the yield was calculated. As aresult, when treatment temperature was 40, 60, and 80° C., the yieldswere 8.5%, 15.4%, and 21.7%, respectively.

EXAMPLE 13 Emulsion Stability Test

[0070] Lecithin powder (Central Soya Co., Centrolex D, USA), Arabic gum(MSC Co., 10308, Korea), and two kinds of dietary fiber extracted fromcorn hulls at 2% (w/w) respectively were added to a mixture of soy beanoil and distilled water, emulsified with a homogenizer at 20,000 rpm for5 minutes, and the viscosity of the emulsion was measured. As a result,the viscosity was 23 cps for lecithin powder, 35 cps for Arabic gum, 55cps for dietary fiber obtained by treating with cellulase andcellobiase, and 50 cps for dietary fiber obtained by treating withcellulase, cellobiase, and xylanse. In addition, the state of emulsionin a 100 mL mass cylinder was investigated. As a result, all the samplesshowed good emulsion stability. After 10 days, a lower aqueous layerseparation occurred for all the samples. After 30 days, a lower aqueouslayer completely separated from the upper oil layer for the sampleincluding lecithin, but the emulsion of the samples including Arabic gumand dietary fiber extracted from corn hulls kept comparatively stable.

What is claimed is:
 1. A method of preparing a soluble dietary fiber comprising the steps of: (i) removing starch and protein from corn hulls; (ii) extracting the starch- and protein-removed corn hulls with an alkaline solution, and filtering the alkaline extract through filter cloth; (iii) treating the filtrate of step (ii) with cellulase and cellobiase; (iv) treating the solution of step (iii) with adsorbent, and filtering it through a membrane filter; and (v) purifying the filtrate.
 2. The method of claim 1, wherein in the step (i), the corn hulls are treated with amylase and protease simultaneously or sequentially.
 3. The method of claim 2, wherein in the step (i), after the treatment of amylase or protease, the resultant is filtered through a filter cloth.
 4. The method of claim 1, wherein in the step (ii), the alkaline solution is a 0.1 to 0.3% of NaOH solution.
 5. The method of claim 1, wherein the step (iii) further comprises a step of treating with xylanase.
 6. The method of claim 1, wherein the cellulase and cellobiase are 0.1 to 5% to the dried corn hulls, respectively.
 7. The method of claim 1, wherein before the step (iii), the alkaline extract from step (ii) is neutralized, and it is desalted and decolorized with an ion exchange resin.
 8. The method of claim 7, wherein the cellulase and cellobiase are 0.1 to 3% to the dried corn hulls, respectively. 